Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, solar cells may be implanted with a dopant species to create an emitter region. This implant may be done using a variety of different mechanisms. In one embodiment, shown in FIG. 1, an ion implant system 100 is used. This ion implant system 100 includes a plasma chamber 105 defined by several walls 107, which may be constructed from graphite, silicon, silicon carbide or another suitable material. This plasma chamber 105 may be supplied with a source gas via a gas inlet 110. This source gas may be energized by an RF antenna 120 or another mechanism to create plasma 150. The RF antenna 120 is in electrical communication with a RF power supply (not shown) which supplies power to the RF antenna 120. A dielectric window 125, such as a quartz or alumina window, may be disposed between the RF antenna 120 and the interior of the plasma chamber 105. The system 100 also includes a controller 175. The controller 175 may receive input signals from a variety of systems and components and provide output signals to each to control the same.
Positively charged ions 155 in the plasma 150 are attracted to the substrate 160 by the difference in potential between the plasma chamber 105 (which defines the potential of the plasma 150) and the substrate 160. In some embodiments, the walls 107 may be more positively biased than the substrate 160. For example, the walls 107 may be in electrical communication with a plasma chamber power supply 180, which is positively biased. In this embodiment, the substrate 160 is in communication with a platen 130, which is in communication with bias power supply 181, which is biased at a voltage lower than that applied by plasma chamber power supply 180. In certain embodiments, the bias power supply 181 may be maintained at ground potential. In a second embodiment, the plasma chamber power supply 180 may be grounded, while the bias power supply 181 may be biased at a negative voltage. While these two embodiments describe either the substrate 160 or the walls 107 being at ground potential, this is not required. The ions 155 from the plasma 150 are attracted to the substrate 160 as long as the walls 107 are biased at any voltage greater than that applied to the platen 130.
During operation, the ions and various forms of neutral particles in the plasma 150 may be deposited on the walls 107 of the plasma chamber 105. In general, deposition layers are poor in electrical conductivity and may even be electrically insulating. As a result, the plasma 150 is not well-referenced, often causing plasma potential changes, non-uniformity of plasma (causing non-uniform doping on substrate 160), and plasma instability. This deposition of material may be uneven and may affect the conductivity of the walls 107. Specifically, the deposition may be uneven, such that some portions of the walls 107 are coated, while other portions remain exposed. This non-uniform coating may affect the composition of parameters of the plasma, which may negatively impact the substrates being implanted.
In order to provide a reliable electrical reference for the plasma 150, it is desirable to insure that no deposition coating exist on the walls 107. However, removing this coating may require the application of large amounts of heat, which may not be practical. Alternatively, the ion implant system may need to be taken offline so that the coating can be removed, which reduces throughput and efficiency.
Therefore, a system and method to provide a reliable electrical reference to the plasma 150 for stable and repeatable doping process by reducing or eliminating coatings that are deposited in a plasma-based implantation system is needed.